A problem presently encountered when testing large VLSI devices based on signature analysis is an occasional occurrence of unstable signatures. Unstable signatures result from indeterminate logic states, referenced to x-states, propagating through the logic circuits and subsequently being captured into the signature compression registers. Logic x-states frequently occur in large, high speed, high power and highly integrated devices, and are usually caused by environmental conditions, process variations and marginal designs. Examples of sources of the indeterminate logic states include signal propagation races, switching delta-I glitches, cross-coupling disturbs and marginal signal levels.
The unstable signature problem, usually found in Multiple Input Signature Registers (MISRs) configurations, becomes critical when automated manufacturing pass/fail testing and device disposition is based on a unique single ‘good’ signature comparison. Furthermore, it is often required to continue testing devices with known x-state conditions using signature analysis methods. To support the test capability, one needs to determine the cause of instability, the range of unstable signatures and provide an appropriate masking for the signature register inputs.
When a long stream of binary data (“0”s and “1”s) is compressed into a fixed length binary signature (e.g., 32-bits) using a software or hardware compression algorithm, the technique is referred to as signature generation. As used in testing, the long stream of binary data can include responses from a logic or array structure over many test cycles and multiple outputs.
There are many compression and signature generation methods and algorithms. A common approach in communications and testing in generating signatures is the use of a Linear Feedback Shift Register (LFSR) with XOR inputs to alter the LFSR state machine sequence depending on the input data. The resulting MISR state, e.g., at the end of a test section, is referred to as the signature. Analyzing and comparing the final signature to the expected ‘good’ machine signature is referred to as the Signature Analysis (SA).
Binary signatures consist of only “0”s and “1”s. The “x” state referrers to the unstable signature can also be a “0” or “1” but it is unstable, namely, it changes states unpredictably and intermittently when executing the same test several times, usually resulting in a different final signature. An unstable signature is often caused by several “x” states being compressed during a signature generation test. The “x” state, as previously described, does not “kill” the signature, but rather it modifies its value to an unexpected state. Since the unstable signature results from the “x” state, any marginal test condition can result in the indeterminate states. Extending the test setup conditions, such as voltage, frequency, temperature, and the like, associated with marginal designs can expose the instabilities. Analyzing the signatures with respect to the above setup test variables can determine the source of instability and potentially optimize and improve the power/performance and operating range of the device.